The present invention relates generally to photolithography, and more particularly, to an improvement in optical proximity correction (OPC) by utilizing an improved scattering bar design, as well as a new method for implementing scattering bars in a mask design.
In photolithographic processing, ever finer geometric patterns corresponding to the circuit components must be imaged from a mask onto a silicon wafer. The patterns used to create such masks are generated utilizing CAD (computer-aided design) programs, this process often being referred to as EDA (electronic design automation). Most CAD programs follow a set of predetermined design rules in order to create functional masks. These rules are set by processing and design limitations. The design rule limitations are typically referred to as “critical dimensions” (CD). A critical dimension of a circuit can be defined as the smallest width of a line or the smallest space between two lines. Thus, the CD determines the overall size and density of the designed circuit.
Of course, one of the goals in integrated circuit fabrication is to faithfully reproduce the original circuit design on the wafer (via the mask). Currently, various optical proximity correction (OPC) techniques are utilized to allow the resulting image to more accurately correspond to the desired target pattern. The OPC can be carried out by means of a simulation program or a rule-based software system.
A common OPC technique, which is widely known, is the use of subresolution assist features, or scattering bars. Scattering bars, or thin lines, are attached between structures in order to improve the sharpness of the imaging in lithography. These lines are so thin that they are not imaged on the substrate themselves. Scattering bars correct for optical proximity effects and have been shown to be effective for increasing the overall process window (i.e., the range of focus and exposure dose variation over which features having some specified CD can be printed consistently, regardless of whether or not the features are isolated or densely packed relative to adjacent features.). Generally speaking, the optical proximity correction occurs by improving the depth of focus for the less dense to isolated features by placing scattering bars near those features. Typically, they are arranged parallel to parts of the structure at predeterminable intervals. The scattering bars function to change the effective pattern density (of the isolated or less dense features) to be more dense, thereby negating the undesirable proximity effects associated with printing of isolated or less dense features. It is important, however, that the mask also contain scattering bars, which cannot be imaged onto the substrate by exposure owing to their small dimensions. But they support the imaging of the main structures.
Notwithstanding the widespread use of scattering bars, there remain problems associated with current scattering bar OPC technology when utilized for patterning feature dimensions. Current scattering bar OPC model may generate scattering bars having long lengths. Scattering bars having long lengths pose an electrostatic discharge (ESD) problem in that electric charge often accumulates on the longer scattering bar. FIG. 1 shows a portion of a mask layout and illustrates the problem of ESD resulting from the application of scattering bars utilizing currently known methods of applying scattering bars to a mask design. As shown, the mask layout includes features 10 and scattering bars 20 having long lengths with electric charge 30 accumulated thereon.
When the accumulated electric charge 30 overloads and discharges, the scattering bar is typically damaged or deformed. This results in an unwanted extra pattern printed out on the substrate. Due to the ESD issue in masks, mask retooling and verification costs are increased as new masks need to be made and the risks of yield losses and wafer scraps are also increased. As device pattern sizes become smaller and smaller, the ESD problem becomes more and more pronounced.
For these reasons and other reasons that will become apparent upon reading the following detailed description, there is a need for a method of providing scattering bars in a mask which overcomes the foregoing problems so as to allow for mask ESD prevention and improved OPC and printing performance.